Only days after U.S. Marine veteran Jon Hammar was thrown into a Mexican prison for carrying an antique shotgun into the country, gangsters in the jail warned him of his likely fate – beheading.

“They threw every threat in the book at me,” Hammar said Thursday in his first public remarks since his release Dec. 21 after more than four months in prison.

“They’d cut my head off, they told my family,” he said.

The gangsters demanded money to let Hammar, 27, remain alive, and the beheading threat was a scare tactic that harkened back to his tours of duty as a U.S. Marine in Iraq.

Mexican authorities arrested Hammar on Aug. 13 at the Texas-Mexico border when he crossed into Mexico in a motor home with a vintage shotgun that once belonged to his great-grandfather. Authorities slapped weapons charges on him. A traveling companion went free.

Hammar’s ordeal, first brought to light in a McClatchy story Dec. 6, sparked outrage in the United States, where fellow Marines demanded his release and members of Congress called for a boycott of Mexican tourism.

The outpouring has left Hammar feeling grateful.

“In America, we have people who care,” he said in a telephone interview from his family home in Palmetto Bay, south of Miami.

“I’m really grateful. But at the same time I kind of expected that from Marines,” he said. “Marines don’t just throw each other under the bus. They look after each other.”

Despite his release just days before Christmas, Hammar only now is recovering from a stomach virus and dehydration that required hospitalization that he blames on the conditions he encountered during his incarceration.

When his father received him at the Texas border, the two began the 22-hour drive to Florida to join his mother for the holidays.

Once they got to Lafayette, La., Jon Hammar began developing a fever, and they spent much of a day in a hospital there. Shortly after arriving in the Miami area, Hammar spent several more days in a local hospital.

“I think it was just an accumulation, just not being in a good environment,” Hammar said.

After Hammar’s parents complained to the U.S. Consulate in Matamoros of the gangster threats against their son, the younger Hammar was moved out of the general prison population to a loosely guarded cage, where he spent months with his ankle handcuffed to a bed frame.

Once physically separated from the gangster inmates, the threatening phone calls to Hammar’s parents stopped, he said.

“Jail is bad no matter where you are,” Hammar said, “and when you add the variables of Mexico and what is going on down there, it just gets worse every corner you turn.”

The prison where Hammar was kept in Matamoros is notorious for being under the control of the dreaded Los Zetas criminal gang, which has used beheadings and mass killings to instill fear in rivals and protect their drug trafficking and criminal activities.

Hammar said prison inmates controlled the facility, not the guards.

“There was a tattoo artist in there, and he said to me, ‘You can die in this place for nothing,’” Hammar said.

Prisoners are “full-blown” mobsters, he said, some of them serving up to triple life sentences.

“I knew not to go around picking fights with anybody,” he said.

Compared to prison guards and judicial employees, however, the gangsters struck Hammar as efficient.

“To me, they were acting more professional than the officials in Mexico. If I needed a translator, and the cartel was talking to me, I had three. But when I’d go to court, they laughed at me when I asked for a translator,” he said.

Meals came sporadically.

“You eat when someone gives you food,” he recalled.

At the time of his arrest, Hammar and his traveling companion were headed to Costa Rica. The Winnebago motor home contained three long surfboards, three short boards and a stand-up board.

Surfing was going to be therapy of sorts for post-traumatic stress disorder.

After enlisting in 2003, Hammar served a four-year battlefield tour, first offering protection to Afghan President Hamid Karzai and later deploying to Iraq, where he took part in the bloody campaign to seize control of Fallujah from heavily armed Islamist militants.

After serving another four years in the Reserves, Hammar was diagnosed with PTSD, voluntarily checking himself into a nine-month treatment program at a facility in California’s Napa Valley.

The journey to Mexico and Central America began shortly after his release.

Hammar said he is still perplexed why U.S. Customs and Border Protection agents did not warn him away from taking the .410-bore Sears & Roebuck shotgun into Mexico.

“There are definitely signs that say, ‘Firearms are illegal.’ But 50 feet from that sign it says, ‘Permitted weapons go to this office,’ yada, yada, yada,” Hammar said. Wanting to do things legally, Hammar said he checked at two border crossings.

First, he went to the Veterans Bridge, a major crossing between Brownsville, Texas, and Matamoros, Mexico. After asking questions there, Hammar then crossed at the nearby Los Indios crossing, obtaining a registration form for the gun.

“I went to two different border crossings and asked a dozen different people, and at the end of the day, I got a smile on their face, you know, here’s the paperwork. Enjoy yourself,” Hammar said.

“At any point, if anyone had told me that (I risked jail in Mexico), I would’ve pawned the gun in a pawnshop in a second,” he said.

Hammar said he wasn’t going to Central America to hunt – indeed, hunting for sport is banned in Costa Rica – but to surf. He took the gun, he said, only if he needed to hunt along the way.

Sen. Bill Nelson, a Florida Democrat, has sought written records from Customs and Border Protection related to Hammar’s crossing with the shotgun, asking for an investigation.

Ardent expressions of concern over the Hammar case came not only from Nelson but also from Rep. Ileana Ros-Lehtinen, a Florida Republican who served as chairwoman of the House Foreign Affairs Committee in the past session and spoke about the case on the floor of the House.

Also active on the case were Reps. Michael Grimm, R-N.Y., and Duncan Hunter, R-Calif., both Marine veterans, who called for a travel boycott of Mexico to pressure for Hammar’s release.